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Formation of the black-hole binary M33 X-7 through mass exchange in a tight massive system


The X-ray source M33 X-7 in the nearby galaxy Messier 33 is among the most massive X-ray binary stellar systems known, hosting a rapidly spinning, 15.65M black hole orbiting an underluminous, 70M main-sequence companion in a slightly eccentric 3.45-day orbit1,2 (M, solar mass). Although post-main-sequence mass transfer explains the masses and tight orbit3, it leaves unexplained the observed X-ray luminosity, the star’s underluminosity, the black hole’s spin and the orbital eccentricity. A common envelope phase1, or rotational mixing4, could explain the orbit, but the former would lead to a merger and the latter to an overluminous companion. A merger would also ensue if mass transfer to the black hole were invoked for its spin-up5. Here we report simulations of evolutionary tracks which reveal that if M33 X-7 started as a primary body of 85M–99M and a secondary body of 28M–32M, in a 2.8–3.1-d orbit, its observed properties can be consistently explained. In this model, the main-sequence primary transfers part of its envelope to the secondary and loses the rest in a wind; it ends its life as a 16M helium star with an iron–nickel core that collapses to a black hole (with or without an accompanying supernova). The release of binding energy, and possibly collapse asymmetries, ‘kick’ the nascent black hole into an eccentric orbit. Wind accretion explains the X-ray luminosity, and the black-hole spin can be natal.

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Figure 1: Evolution of the orbital and stellar parameters of M33 X-7.
Figure 2: Progenitor properties and current luminosity.

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We thank N. Ivanova, A. Heger, A. Cantrell and C. Bailyn for discussions during the development of this project.

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Authors and Affiliations



V.K. and B.W. designed the study. F.V. led the project, performed the single and binary star evolution calculations, and developed the code to perform the orbital evolution after black-hole formation. V.K. and B.W. collaborated with F.V. in each step of the project. E.G. maintained, updated and extended the stellar evolution code used, and collaborated with F.V. in performing the calculations. W.M.F. determined the correction to the star’s luminosity and surface temperature due to the inclination of the system. T.F. led the theoretical analysis of the black-hole spin. J.A.O. performed the analysis of the observational data on M33 X-7 using the ELC code for the full distance uncertainty. J.L. recalculated the black hole’s spin at different distances. All authors discussed the results and made substantial contributions to the manuscript.

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Correspondence to Francesca Valsecchi.

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The authors declare no competing financial interests.

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Valsecchi, F., Glebbeek, E., Farr, W. et al. Formation of the black-hole binary M33 X-7 through mass exchange in a tight massive system. Nature 468, 77–79 (2010).

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